1) Field of the Invention
The present invention relates to a technology of providing a light source for use in a reading device such as a digital copying machine or a general purpose scanner.
2) Description of the Related Art
FIG. 18 is a drawing that depicts one example of an image forming apparatus having a document reading device.
In this figure, reference numeral 21 is a photosensitive drum, 22 is a charging device, 23 is a cleaning device, 24 is a developing device, 25 is a transferring device, 26 is a fixing roller, 27 is a pressure roller, 28 is a fixing belt, 29 is a tension roller, 30 is a paper-feeding tray, 31 is a paper-feeding roller, 32 is a transporting roller, 33 is a resist roller, 34 is a paper-discharging roller, 35 is a paper-discharge tray, 90 is an exposing device and 100 is a reading device.
The photosensitive drum 21 is charged on its surface by the charging device 22, while rotating in the sub-scanning direction. The photosensitive member 21 is subjected to a scanning process in the main scanning direction that is perpendicular to the paper face with light L bearing image information that is released from the exposing device 90 to be exposed so that an electrostatic latent image is formed thereon. This electrostatic latent image is visualized by toner from the developing device 24 so that a toner image is formed.
A sheet of copy paper P, located at the uppermost position of the paper-feeding tray 30, is sent toward the paper-feeding roller 32 by the paper-feeding roller 31, and made in contact with the resist roller 33 to be stopped. In synchronized timing with the formation of the toner image on the photosensitive member, the copy paper is sent from the resist roller, and the toner image is transferred thereon by the transferring device 25 so that the toner image is fixed between the fixing roller 26 and the pressure roller 27, and the resulting copy paper is discharged onto the paper-discharge tray 35 by the paper-discharging roller 34, etc.
FIG. 19 is a schematic block diagram that depicts the reading device.
In this figure, reference numeral 1 is a contact glass plate that forms a document platen, 2 is a first carriage, 3 is a second carriage, 4 is an image forming lens, 5 is a CCD that serves as an image reading element, 6 is a light source, 7 is a first mirror, 8 is a second mirror and 9 is a third mirror.
A document, not shown, is put on the contact glass plate 1, with its document surface facing down, and a light source is turned on, and the first carriage 2 is shifted in the direction of arrow A at a predetermined speed. In this case, the second carriage 3 is shifted in the direction of arrow B at half the speed of the first carriage 2. The image forming lens 4 is placed with such a positional relationship that the image on the document surface is converged and formed as an image on the CCD 5 through the first to third mirrors 7, 8 and 9.
Even during the carriage shift, the surface of the contact glass plate 1 (document face) and the CCD 5 are allowed to always maintain their conjugate relationship with respect to image forming lens 4 so that a clear image is always taken by the CCD 5.
FIG. 20 is a drawing that depicts the conjugate relationship between the document surface and the CCD 5.
In this figure, codes R, G, B on the CCD represent reading-use CCD lines used for respective three primary colors (red, green, blue), and codes R, G, B on the document surface represent reading positions corresponding to the respective CCD lines, respectively.
Supposing that the reading width of the CCD is represented by a and that the gap of the respective lines of the CCD is b, the illumination light requires at least a width of 2b +a in the sub-scanning direction, when converted as the position of CCD. Supposing that the image forming magnification by the image forming lens is represented by m (m<1), the illumination width C on the document surface needs to satisfy at least the following equation: C=(2b+a)/m. When production errors and the like are taken into consideration, this value is preferably set to a slightly greater value; however, when the value becomes too great, the quantity of light that is not utilized effectively becomes greater, resulting in degradation in lighting efficiency.
FIG. 21 is a drawing that depicts an illuminance distribution in the sub-scanning direction on the document face, which is formed by a conventional light source.
In this figure, reference numeral 10.is an opposing mirror, 11 represents a distribution curve of direct light derived from a light source and the opposing mirror when there is no document, and 12 represents a illuminance distribution curve on an actual document face including secondary reflection and the like when white paper is placed.
Conventionally, a discharge tube such as a fluorescent lamp and a xenon lamp has been used as a light source, and this case has a structure as shown in this figure.
The illuminance distribution curve 11 is distributed virtually symmetrically with respect to the range C; however, the illuminance distribution curve 12 has a distribution that is biased slightly to the right. The reason for this is presumably because the illumination range of the light source 6 is wide so that light is also made incident on the document face other than the above-mentioned range C, with the result that light reflected from the document face hits the tube wall of the light source 6 to be again reflected to illuminate the document at a portion right above the light source.
The illuminance distribution curve 12 of this type causes not only a problem of loss of quantity of light, but also another problem when a compatible-type scanner of the fixed document type and the document-shifting type is used.
FIG. 22 is a drawing that depicts the illuminance distribution of the compatible-type scanner.
In this figure, reference numeral 13 represents ADF-use contact glass, 14 and 15 are opaque guide members, 16 and 17 are document feeding rollers, 18 is a sheet-shaped document, and 19 represents an illuminance distribution curve on the ADF side.
As shown in FIG. 19, upon reading the fixed document, the first and the second carriages are shifted. In contrast, upon reading the shifting document, the sheet-shaped document 18 is sent to the surface of the ADF-use contact glass 13 by the document feeding rollers 16 and 17 so that the image, illuminated by the light source 6 and the opposing mirror 10, is read by the CCD 5 through an image forming optical system including the first mirror 7 and the like.
This figure depicts the illuminance distribution curve at the position for use in the shifting document and the illuminance distribution curve at the position for use in the fixed document in a combined manner.
The illuminance distribution curve 12 at the position for use in the fixed document is the same as the distribution shown in FIG. 21. In contrast, with respect to the illuminance distribution curve 19 at the position for use in the shifting document, since the portion right above the light source 6 is virtually covered with the guide member 15 that is an opaque member, there is no reflection from the corresponding area and consequently, there is no unnecessary re-reflection. For this reason, there is virtually no illuminance distribution in an area outside the ADF contact glass 13; therefore, the illuminance distribution curve 19 forms a shape very close to the illuminance distribution curve 11 derived from direct light, shown in FIG. 21.
The difference in these illuminance distribution curves cause no problems when individually used; however, when a compatible-type scanner is formed by using the same CCD, two problems are newly raised.
One of the problems is that the quantity of illumination light differs depending on document faces, with the balance of quantities of light being different depending on colors. In comparison with the illuminance distribution curve 19 that has virtually a symmetrical shape with respect to the illumination range C, the illuminance distribution curve 12 has a great quantity of light as a whole with the peak value being biased to the light. Therefore, when the same CCD is used for receiving light while changing the reading systems, the sensitivity to the quantity of light and the sensitivity balance with respect to colors need to be changed. The sensitivity to the quantity of light can be electrically, controlled by using the AGC; however, the change in the sensitivity balance with respect to different colors needs to be taken into consideration in a separate manner.
Another problem is raised when, in the case of the fixed document, the document has a great difference in light and dark portions in the sub-scanning direction of the document. In other words, the reason that the peak value of the illuminance distribution curve 12 is biased to the right is because the reflected light from the document face is again reflected by the light source tube wall. Therefore, while the light portion of the document is located right above the tube wall, the illuminance distribution curve 12 as shown in FIG. 22 is obtained; however, while the dark portion of the document is located right above the tube wall, the above-mentioned re-reflection is no longer exerted, the corresponding characteristic tends to form a curve that is close to the illuminance distribution curve 19. Here, the AGC should not be applied to the change of this type immediately. The reason for this is because, since the dark portion of the document successively enters the reading range, the immediate application of the AGC fails to recognize this document portion as a dark portion accurately, resulting in a failure in carrying out the faithful image reproduction.
In order to solve the problem of re-reflection of light that has once hit the document face, a method has been proposed in which a ratio of quantities of light within a predetermined range is specified (for example, see page 2, paragraphs 0007 to 0010, and FIG. 6 of Japanese Patent Application Laid-Open No. 2001-222076). In this method, although it is possible to solve the re-reflection from the document platen glass, no solution is given to the problem of the re-reflection from the tube wall of a Xe lamp or the like used as the light-source.
Another arrangement has been proposed in which the illumination light is converged to a narrow range so that the necessity of having to consider the illuminance other than the reading range is eliminated (for example, see page 4, paragraphs 0025, and FIG. 3 of Japanese Patent Application Laid-Open No. 2002-125098). Although these arrangements are superior in preventing the generation of re-reflection, these arrangements generally cause a chevroned illuminance distribution unless a special contrivance is prepared, with the result that, in the case of a color-image reading process using a three-line CCD, another problem is raised in that the luminances of the three colors are not made equal to one another.